If F is the same but m is different, what happens to a?

Buoyancy is functionally irrelevant here. Buoyancy in air effectively subtracts 1.3kg per cubic meter of each substance: The mass of the volume of air displaced by the object.

The part you are not understanding: Drag applies the same force to both objects. Gravity applies the same acceleration to each object.

Drag doesn't exist in a vacuum.

They did. You didn't understand what they said.

Two items of the same shape will have the same amount of air resistance. If they have significantly different masses, the two object experience commensurately different accelerations (or reduction in acceleration), even if the force is the same.

The "same force" they are talking about is drag. The two objects are the same size and shape. At the same velocity, drag affects them both equally, applying an equal, upward force against both objects.

Gravity (in a vacuum) accelerates both objects equally. But they have differing masses. F=MA. F/M = A. A is equal for both objects. Because acceleration is equal, the "force" on each object is not: the force must be proportional to its mass: The high mass object must be experiencing high force; the low-mass object must be experiencing low force.

Subtract the "same force" of drag from the downward force on both objects, and the net force on each object is no longer proportional to the mass of each object. Consequently, the high-mass object accelerates in atmosphere faster than the low-mass object. The high-mass object has a higher terminal velocity; the low-mass object has a lower terminal velocity.

For the purposes of this experiment, buoyancy is functionally irrelevant. The effect of buoyancy is to subtract a fixed mass from each object: A mass equivalent to the mass of air displaced by the object. Effectively, buoyancy slightly reduces the density of both objects. The actual difference in the densities of the two objects is far greater than the slight change due to buoyancy in air, so buoyancy is not a significant factor.